The present invention relates generally to an image recording apparatus for recording an image on a photosensitive member. More particularly, the invention relates to an exposure device wherein the photosensitive member receives a constant amount of exposure despite there is a variation in a moving speed of the photosensitive member.
Heretofore, there has been known an image recording apparatus having an arrangement shown in FIG. 1. A drum 2 is rotatably supported on a frame (not shown) to be rotatable in the counter-clockwise direction as indicated by an arrow. A photosensitive sheet 1 is wound around the peripheral surface of the drum 2. A charging station 3, a laser scanner 4, a developing station 5 and a discharging station 6 are disposed around the periphery of the drum 2.
In operation, the photosensitive sheet 1 is supplied onto the drum 2 from a sheet supply unit (not shown) and is held on the peripheral surface of the drum 2 by means of a suction unit (not shown). The photosensitive sheet 1 placed on the drum 2 is uniformly charged in the charging station 3. A laser beam emitted in accordance with image data is scanned by the laser scanner 4 and is applied onto the photosensitive sheet 1. Thus, the photosensitive sheet 1 is exposed to the laser beam and a latent image is formed thereon. The latent image is developed in the developing station 5, thereby forming a visible image on the photosensitive sheet 1.
As shown in FIG. 2, the drum 2 is rotated by a motor 7 via speed reduction gears 8a, 8b. An encoder 9 is provided in the rotary shaft of the motor 7 to detect a rotational speed of the motor 7. The data regarding the rotational speed of the motor 7 are fed back to a motor drive controller 10 so that the motor 7 is rotated at a constant speed.
However, notwithstanding the fact that the motor 7 rotates at a constant speed, the actual rotational speed of the drum 2 varies in the range of several to several tens hertz due to the backlash of the gears 8a, 8b and/or the variation in the load imposed on the drum 2. Therefore, the auxiliary scanning pitch in the circumferential direction of the drum 2 changes depending on the drum speed variation, whereby an amount of exposure per a unit area on the photosensitive sheet 1 changes depending on the drum speed variation. The amount of exposure is defined by a product of the laser beam intensity and an exposure time. For a prescribed peripheral speed V (m/sec) of the drum 2 with a variation .DELTA.V, the exposure time T (sec) at which a point on the photosensitive sheet 1 is exposed to the laser beam is expressed by: EQU T=T.sub.o .multidot.V/(V+.DELTA.V)
where T.sub.o represents the exposure time when there is no variation in the drum speed.
For the laser beam intensity P(watt), the exposure amount E is given by: ##EQU1##
To summarize, even if it is intended to uniformly expose the entire surface of the photosensitive sheet 1 to the laser beam, the amount of exposure changes depending on the rotational position of the drum 2 due to the drum speed variation. When the drum 2 is instantaneously at a speed higher than the prescribed regular speed, the auxiliary scanning pitch is widened and thus the exposure amount is reduced. On the other hand, when the peripheral speed of the drum 2 is subsequently lowered, the exposure amount is increased. Therefore, the visible image obtained by developing the latent image is not uniform in density. As such, there is a problem in the conventional image recording apparatus such that the image quality is greatly degraded.